Wireless networks have become increasingly popular, as computers and other devices can be coupled for data communications without requiring wired connections between the network nodes. One set of standards for wireless networks is the IEEE 802.11 standards, but other wireless standards or protocols might be used instead.
Because wireless networks are expected to operate in unfavorable conditions, such as in the presence of reflections, interference, movement of receivers/transmitters, etc., much effort is needed to correctly transmit and receive data over a wireless channel. In addition, since many wireless devices are expected to be portable and/or mobile, the applications are often constrained in terms of power consumption and computing power.
A typical node in a wireless network (referred to in the standards as a “station”) includes a receive chain and a transmit chain. A transmit chain typically includes some digital processing and analog, RF circuitry that causes a signal to be transmitted into the wireless channel. A receive chain typically includes one or more antenna, RF and analog circuitry, and digital processing that seeks to output a data stream that represents what the sending transmit chain received as its input and transmitted into the wireless network.
A typical node in a wireless network includes a receive chain and a transmit chain and each chain uses only one antenna at a time. However, with multiple input, multiple output (MIMO) communication systems, more than one transmitter antenna and/or more than one receiver antenna is used, with each transmitter antenna possibly transmitting different bitstreams as other transmitter antennas and each receiver antenna preferably receiving at least a slightly different input from the channel than other receiver antennas.
MIMO communication systems are known in the art. Such systems generally include a transmitter having a number (MT) of transmit antennas communicating with a receiver having a number (Mr) of receive antennas, where Mr and Mt may or may not be equal. In some keying schemes, bits of data to be transmitted are grouped and each group of bits is mapped to a symbol (a particular combination of phase and amplitude) in a signaling constellation. A number of constellations are known in the art, including binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM) constellations. In a MIMO communication system, each of the Mt transmit antenna transmits, at substantially the same time, a symbol representing a different group of bits. Thus, if each symbol represents B bits, the number of bits transmitted per channel “period” is B*Mt.
Each receive antenna receives a signal that is a combination of signals from the transmit antennas, modified by channel properties (e.g., fading and delay), noise, interference (intentional or unintentional) from other devices or objects radiating into the channel in range of the receiver. The receiver decodes (i.e., reconstructs) the Mt transmitted signals from the Mr received signals using its knowledge of the possible transmitted symbols and the properties of the communication channel. Because of the improved reception abilities of multiple antenna systems, they are often expected to receive signals with lower signal-to-noise ratios (SNR) than other systems. With a wider expected operating range in terms of SNR, correct packet detection is expected at lower SNRs, making many conventional packet detection schemes unsuitable.
Wireless networks are typically designed with layers, such as the seven networking layers known as the ISO/OSI model. The lowest of these layers is the PHY (physical) layer, concerned with transmitting signals. The next layer that interfaces the PHY layer with higher-level layers is the MAC (medium access control) layer.
The 802.11 MAC layer provides for Carrier-Sense-Multiple-Access (CSMA) protocols for time-division-multiplexing of data traffic. In such a network, data traffic is organized in packets. With CSMA, each radio checks the wireless medium to see if it is being used by others (i.e., if there are others transmitting packets) before using it. As a consequence, it is important that each device be able to accurately measure whether another device is using the medium or not, to avoid interfering with those other devices. Therefore, an 802.11 receiver must be designed with a packet detector or logic that attempts to determine when the medium is being used. A packet detector might also be used to trigger the demodulation and decoding of signals to recover packet data when a packet is detected, allowing the demodulation or and decoder or such logic to remain quiescent when no packets are present.
Typical packet detectors use an increase in signal strength seen by the demodulator as an indicator that a packet has arrived. Another method commonly used is to correlate the incoming samples from the medium with the expected preamble signal sent with every 802.11 packet. Another method used is to auto-correlate the incoming samples to determine if they have cyclic properties similar to an 802.11 packet preamble.
A “false detection” is defined as an event wherein the packet detector determines that a packet exists on the medium when in fact no packet is present. When a false detection occurs, the performance of the transceiver is severely affected. A false detection means that a carrier sense mechanism would falsely measure the channel as being busy, causing the radio's transmitter to unnecessarily defer (i.e., hold off on sending any packets into the medium) and the radio's receiver might also remain blind to any true packets that arrive from the medium. This reduces throughput and causes performance degradation due to missed packets and missed opportunities to send packets.
False detections can occur due to bursty high levels of noise from the local environment such as the laptop electronics (CPU, Ethernet controllers) which appear like packets received near the noise floor (low SNR packets). False detections can also occur due to interference from other non-802.11 devices in the same channel (e.g., Bluetooth devices). These sources of interference and noise are typically time-varying, creating additional problems.
Similar techniques might be used with non-standard 802.11 systems and non-802.11 systems, with similar shortcomings.